The present invention relates to an optical recording medium such as an optical disc, and a method for manufacturing the optical recording medium. More particularly, the present invention relates to a write-once type optical disc on which data can be recorded using an organic dye, or the like.
Compact Discs (CD) have achieved widespread use, and optical discs have already won a position as important recording media. In addition to a CD of read-only type, the widespread use of a CD-R as a disc of write-once type on which information can be recorded is remarkable. In recent years, research and development on high-density optical discs are intensively performed, and a DVD of higher density than that of a CD is proposed and practically used. Among the standards of the DVD, a DVD-R which is a write-once type optical disc is expected as a medium which is relatively inexpensive and on which information can be recorded. In addition, development on higher-density optical discs for recording signals with further higher density for high-definition TV in the future or the like which require a large capacity is pursued.
Conventionally, the write-once type optical disc, typified by CD-R, mainly uses a recording material including an organic dye as a primary component. In the case of DVD-R, substantially the same recording material is used.
Hereinafter a conventional configuration of a write-once type optical disc will be described by using an example of DVD-R.
FIG. 2 illustrates a sectional configuration of a conventional DVD-R disc. The shown DVD-R disc has a configuration in which a first base plate 201 and a second base plate 204 are bonded together by means of an adhesion layer 205. In the first base plate 201, a guide tracking groove 201a is formed in a first surface 201c, and a second surface 201d is a mirror face. FIG. 2 illustrates a plurality of cross sections of a single spiral form of the groove 201a. The first base plate 201 is formed by injection molding. The shape of the first surface 201c is transferred from a stamper.
On the first surface 201c of the first base plate 201, a recording layer 202 containing an organic dye, and a reflection layer 203 are stacked. The recording layer 202 is applied by spin coating, and the reflection layer 203 is deposited on the recording layer 202 by a method such as sputtering. The inside of the groove 201a of the first base plate 201 is filled with the applied recording layer 202, so that a surface of the recording layer 202 is substantially flat, regardless of the unevenness of the first surface 201c of the underlying recording layer 202. Accordingly, a surface of the reflection layer 203 deposited on the recording layer 202 is substantially flat.
The second base plate 204 is formed by injection molding, similarly to the first base plate 201. The second base plate 204 is bonded, via an adhesion layer 205, to the first base plate 201 in which the recording layer 202 and the reflection layer 203 are stacked.
Writing/reading radiation or light is converged by an objective 206 on an optical head, and is applied onto an optical disc from the side of the second surface 201d of the first base plate 201. More specifically, the groove 201a is irradiated with the writing/reading radiation that is transmitted through the first base plate 201.
Next, with reference to FIG. 3, another example of an optical disc will be described.
The optical disc of FIG. 3 includes an organic dye type recording layer 302 formed by deposition instead of spin coating. In the optical disc, since the recording layer 302 is deposited, a surface of the recording layer 302 has a shape reflecting a shape of a groove or a pit formed on a first surface 301c of a first base plate 301. The shape is reflected in a surface of a reflection layer 303. To the optical disc, writing/reading radiation is applied from the side of a second surface 301d of the first base plate 301.
In both of the above-described optical discs, tracking control is performed so that the writing/reading radiation tracks the groove. Usually, the detection of the tracking deviation is performed by a push-pull method. In the push-pull method, reflected radiation from a disc is converged on two detectors divided along a line which is parallel to a groove direction, and a difference in intensities of radiation detected by the right and left detectors is detected as a tracking signal. Due to a phase difference between reflected radiation from a groove and reflected radiation from a land between grooves, the signal is zero when a converged radiation beam spot of the writing/reading radiation is in a center position of the groove or the land. When the converged radiation beam spot is positioned between the center of the groove and the center of the land, the signal is a positive value or a negative value. When a wavelength of the writing/reading radiation is xcex, and k is 0 or a natural number, an amplitude of the tracking signal is the maximum in a condition where an absolute value (unit: radian) of a phase difference between reflected radiation from a groove and a land is xcfx80(2k+1)/2.
In the conventional optical disc shown in FIG. 2, the recording layer 202 is formed by spin coating, so that there is an advantage that the period of time required for the step of forming the recording layer 202 can be shortened. However, since the surface of the reflection layer 203 is formed so as to be substantially flat, the following drawback arises. That is, a phase difference xcex4 of reflected radiation through a groove portion and reflected radiation through a land portion is represented by an equation of xcex4=4xcfx80xc3x97((n1xe2x88x92n2)xc3x97d1+n2xc3x97d2)/xcex, when a diffraction index of the first base plate 201 is n1, a diffraction index of the recording layer 202 is n2, a depth of a groove is d1, and a surface step of the recording layer 202 between a groove portion and a non-groove portion is d2.
When d2 is small and a difference between the diffraction index n1 of the first base plate 201 and the diffraction index n2 of the recording layer 202 is small, in order to obtain a sufficient amplitude of the tracking signal, it is necessary to increase the groove depth d1 for generating a phase difference. As a result, for the purpose of transferring a deep groove or pit, the period of time is elongated because a temperature of resin and mold is set high when the base plate is formed by injection molding. In addition, it is difficult for the molded base plate to be released from the stamper, and a mold release non-uniformity and a cloud may easily occur. Thus, it is difficult to improve the productivity.
In the case of the optical disc of FIG. 3, the recording layer 302 is formed by deposition, so that the reflection layer 303 is formed in accordance with the shape of the groove of the base plate 301. As a result, the phase difference xcex4 is represented by an equation of xcex4=4xcfx80xc3x97n1xc3x97d1/xcex. Thus, a sufficient phase difference can be attained, and a good tracking signal amplitude can be obtained without increasing the groove depth d1.
However, the formation of a film by depositing a recording material containing an organic dye is slow, and it is difficult to improve the productivity.
Moreover, in both of the optical discs, since the writing/reading radiation is applied through the base plate in which grooves or pits are formed, it is important to fabricate the base plates 201 and 301 in which grooves or pits are formed with high precision and good reproducibility. The thickness of the respective base plates 201 and 301 affects the recording density. Generally, in order to improve the recording density of an optical disc, it is necessary to reduce a beam spot diameter of laser radiation used for writing/reading. For this purpose, it is necessary to shorten the wavelength of the laser radiation and to adopt an optical system having a high NA (Numerical Aperture) value. However, when the NA value is large, an aberration due to tilt of disc is increased, and the reproduction signal is deteriorated. In order to reduce such an aberration, it is preferred that the thickness of the respective base plates 201 and 301 be reduced. However, since the base plates 201 and 301 are formed by injection molding, it is difficult to precisely form the shapes of grooves or pits while reducing the thickness thereof. Thus, in the optical discs of FIG. 2 and FIG. 3, it is difficult to further improve the recording density.
The invention provides an optical disc in which a sufficiently good signal can be obtained even when the depth of a groove for tracking is reduced, and which is suitable for improving the recording density, and a method for fabricating the optical disc.
The optical disc of the present invention is an optical disc comprising a first base plate having a surface in which a recess is formed and a multi-layer structure disposed on the surface, wherein the multi-layer structure includes: a reflection layer disposed on the surface of the first base plate; a recording layer, formed on the reflection layer, for substantially filling an inside of the recess; and a transparent member, disposed for covering the recording layer, which is capable of transmitting writing/reading radiation.
In a preferred embodiment, a thickness of the recording layer in a bottom portion of the recess of the first base plate is 1.5 times or more as large as a thickness of the recording layer in a flat portion of the first base plate.
In a preferred embodiment, a top surface of the recording layer is substantially flattened, whereby a step in the top surface of the recording layer being smaller than a step in a top surface of the reflection layer.
In a preferred embodiment, the recording layer is formed by application.
In a preferred embodiment, the recording layer is made of a recording material containing a dye.
In a preferred embodiment, the recording layer is partially in contact with a surface of the first base plate via an opening portion existing in the reflection layer.
In a preferred embodiment, the first base plate is made of a material that reacts with a recording layer irradiated with writing radiation.
In a preferred embodiment, the reflection layer is made of a material having an island-like structure.
In a preferred embodiment, the transparent member is made of a radiation curable resin.
In a preferred embodiment, the transparent member is made of a thermosetting resin.
In a preferred embodiment, the transparent member includes: a second base plate capable of transmitting the writing/reading radiation; and an adhesion layer for bonding the second base plate to the first base plate, and the first base plate and the second base plate are bonded together via the adhesion layer.
In a preferred embodiment, the adhesion layer is made of a radiation curable resin.
In a preferred embodiment, the transparent member includes a semitransparent reflection layer and a second recording layer.
In a preferred embodiment, in the second recording layer, read only information is recorded or stored in the form of a groove and or a pit.
In a preferred embodiment, a protection film capable of transmitting the writing/reading radiation is formed between the first base plate and the adhesion layer.
In a preferred embodiment, a thickness of the transparent member is 0.3 mm or less.
In a preferred embodiment, a thickness of the first base plate is 1.0 to 1.2 mm.
In a preferred embodiment, a depth of the recess is xcex/(4xc3x97n) or less, when a wavelength of reading radiation is xcex and a refractive index of the recording layer is n.
The method for fabricating an optical disc of the present invention is a method comprising a step of producing a first substrate, a step of producing a second substrate, and a step of bonding the first and second substrates by an adhesive that is substantially transparent, wherein the step of producing the first substrate includes the steps of: (a) preparing a base plate provided with a surface having a recess; (b) forming a reflection layer on the surface of the base plate; and (c) substantially filling an inside of the recess with a recording layer.
In a preferred embodiment, a thickness of the recording layer in a bottom portion of the recess of the base plate is 1.5 times or more as large as a thickness of the recording layer in a flat portion of the base plate.
In a preferred embodiment, the step (c) includes a step of applying the recording layer on the reflection layer by spin coating.
In a preferred embodiment, the step of producing the second substrate includes: a step of forming a pit and or a groove on one side of a base plate that is substantially transparent; and a step of forming a semitransparent film.
In a preferred embodiment, a radiation curable resin is used as the adhesive that is substantially transparent.
In a preferred embodiment, after the application step, a protection film is formed on the recording layer, and a bonding step is performed.
The method for fabricating an optical disc of the present invention is a method comprising the steps of: (a) preparing a base plate provided with a surface having a recess; (b) forming a reflection layer on the surface of the base plate; (c) substantially filling an inside of the recess with a recording layer; (d) applying a material that is substantially transparent; and (e) curing the substantially transparent material.
In a preferred embodiment, a thickness of the recording layer in a bottom portion of the recess formed in a surface of the reflection layer is 1.5 times or more as large as a thickness of the recording layer in a flat portion of the surface of the reflection layer.
In a preferred embodiment, the step (c) includes a step of applying the recording layer on the reflection layer by spin coating.
In a preferred embodiment, the method further includes after the application step and before the step of applying the substantially transparent material: a step of forming information layer in which information is recorded in the form of a pit or a groove; and a step of forming a semitransparent reflection layer.
In a preferred embodiment, the application of the substantially transparent material is performed by spin coating.
In a preferred embodiment, the substantially transparent material is made of a radiation curable resin, and the substantially transparent material is irradiated with radiation in the step of curing the substantially transparent material.